ALSA: usb-audio: Adjust Gamecom 780 volume level

Original patch fixed the original problem, but the sound was far too low
for most users. This patch references a compare matrix to allow the
volume levels to act normally. I personally tested this patch myself,
and volume levels returned to normal. Please see this discussion for
more details: https://bugzilla.kernel.org/show_bug.cgi?id=65251

Signed-off-by: Paul S McSpadden <fisch602@gmail.com>
Cc: <stable@vger.kernel.org> [v3.14+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>